1. Field of the Invention
The present invention relates to a writing instrument. It should be noted that, in the invention, the term “front” refers to a closed side of a cap and a pen tip side of a writing instrument body, whereas the term “rear” refers to an open side of the cap and an ink tank side of the writing instrument body.
2. Description of the Related Art
Conventionally, among the writing instruments of this type, as a cap for a writing instrument which is safe even if an infant swallows it, Japanese Patent Unexamined Publication JP-A-9-39479 discloses “a cap for a writing instrument characterized in that a seal portion for sealing a pen body by coming into close contact with a writing instrument body is formed on an inner peripheral surface of an opening portion of a bottomed cylindrical cap body, that a plurality of  longitudinal ribs are formed on an outer periphery of the cap body to allow gaps between the longitudinal ribs to act as vent grooves, that a tailing end part of a skirt cylinder whose diameter is the same as that of the writing instrument body is connected to the cap body by connection ribs in the vicinity of the opening portion of the cap body shunting the seal portion, and that venting windows communicating with the vent grooves are formed between the connection ribs.”
In addition, as a structure for discharging the air compressed and entrapped in a cap at the time of fitting the cap, Japanese Patent Unexamined Publication JP-A-10-100585 discloses “a structure for fitting a cap of a writing instrument concerning a cap formed by an inner cylinder portion for accommodating a pen tip and an outer cylinder portion having an annular retaining projecting edge formed on an upper portion of an inner wall surface and a retaining projection provided projectingly on a lower portion thereof. A vent hole communicating with the interior of the outer cylinder portion is formed in a bottom portion of an annular U-groove formed between the inner cylinder portion and the outer cylinder portion in an axial direction of a writing instrument. The inner wall surface of the outer cylinder portion of the cap is  fitted to a writing instrument body. The writing instrument body has a bellows portion (corresponding to an ink reservoir member of this application) for temporarily storing excessively supplied ink. A writing portion is fixedly held which directly supplies ink to the pen tip through a supply cylinder communicating with the incorporated ink reservoir portion. An annular resilient member which is deformed by close contact with a bottom of the inner cylinder portion is provided on an upper edge of the writing instrument body, thereby rendering the vent hole of the cap closable.”
The cap of the above-described Japanese Patent Unexamined Publication JP-A-9-39479 is so structured that a seal portion for sealing the pen body by coming into close contact with the writing instrument body is formed on the inner peripheral surface of the cap body (corresponding to the inner cylinder portion in this application). Therefore, when the cap is fitted, there is a possibility of the pen tip coming into contact with the seal portion, with the result that ink can attach to and smear the seal portion or can damage it, possibly causing the occurrence of faulty airtightness.
Particularly in the case of the cap disclosed in the above-described Japanese Patent Unexamined  Publication JP-A-9-39479, if the ink is attached to the seal portion, there are possibilities that the ink attached to the seal portion becomes attached to the writing instrument body by the fitting of the cap, and that when the writing instrument body is gripped, the ink is attached to the hand, smearing the hand.
On the other hand, the cap fitting structure disclosed in the above-described Japanese Patent Unexamined Publication JP-A-10-100585 is so structured that an annular resilient member, which is a separate part, is provided on an upper edge of the writing instrument body, so that the parts and the manufacturing steps increase. As a result, the manufacturing cost increases, and it becomes difficult to provide the writing instrument at low cost.
In addition, in the first aspect of the present invention, if the annular seal portion is arranged to be provided on an outer peripheral surface of the inner cylinder portion rearwardly of the connection ribs, when during the molding of the cap, a core of a mold for molding the connection ribs and the annular seal portion is pulled out rearwardly from the cap after molding, connection-rib molding portions of the core of the mold can come into contact with the annular seal portion on the outer peripheral surface of the inner  cylinder portion and damage the annular seal portion. Hence, there is a possibility that proper sealing characteristics cannot be obtained between the annular seal portion and the annular airtight portion of the writing instrument body.